Abstract: Semiconductor devices continue to press into the nanoscale regime, and newapplications have emerged for which the quantum properties of dopant atoms actas the functional part of the device, underscoring the necessity to probe thequantum structure of small numbers of dopant atoms in semiconductors1-3.Although dopant properties are well-understood with respect to bulksemiconductors, new questions arise in nanosystems. For example, the quantumenergy levels of dopants will be affected by the proximity of nanometer-scaleelectrodes. Moreover, because shallow donors and acceptors are analogous tohydrogen atoms, experiments on small numbers of dopants have the potential tobe a testing ground for fundamental questions of atomic and molecular physics,such as the maximum negative ionization of a molecule with a given number ofpositive ions4,5. Electron tunneling spectroscopy through isolated dopantshas been observed in transport studies6,7. In addition, Geim and coworkersidentified resonances due to two closely spaced donors, effectively formingdonor molecules8. Here we present capacitance spectroscopy measurements ofsilicon donors in a gallium-arsenide heterostructure using a scanning probetechnique9,10. In contrast to the work of Geim et al., our data showdiscernible peaks attributed to successive electrons entering the molecules.Hence this work represents the first addition spectrum measurement of dopantmolecules. More generally, to the best of our knowledge, this study is thefirst example of single-electron capacitance spectroscopy performed directlywith a scanning probe tip9.